DESCRIPTION: The effects of retrieval practice on learning and memory have important implications, yet they have been investigated relatively little, and scarcely at all outside the narrow domain of verbal learning. Some data suggest that retrieval practice (without forced responding and with delayed correction) enhances learning more than the same amount of time devoted to additional study (Carrier and Pashler, 1992). If this principle has broad generality, it could be exploited to optimize training in many fields including medicine (e.g., mastering disease signs and symptoms, radiological diagnosis, etc.). New experiments and computational modeling are proposed to determine under what conditions retrieval practice increases learning, to assess the generality of testing effects beyond verbal learning, and to explore implications for underlying neural-computational mechanisms of learning. Testing inevitably leads to errors, but the consequences of errors have been little investigated. Edwin Guthrie argued that errors "stamp in" undesirable learning even if the person immediately realizes s/he made a mistake. Experiments will examine this intuitively appealing but untested notion, contrasting it with the alternative suggestion that errors may promote learning. A final set of experiments explores "fading" in perceptual discrimination training, starting with exaggerated differences and gradually narrowing the gap, thereby minimizing errors. A few studies suggest that fading speeds acquisition, but its power and generality are unknown. Experimental and computational studies are outlined to determine the possible benefits of fading and explore their possible implications for neural-computational bases of learning.